Essentially two types of sludge occur in municipal and in part also industrial wastewater treatment plants. These two types are the primary or preliminary sedimentation sludge, hereinafter termed primary sludge, and the biological secondary or excess sludge, hereinafter termed excess sludge. In accordance with the current procedure, the chemical sludges scarcely any longer occur separately, but are integrated in the biological sludge by a simultaneous precipitation.
The primary sludge is generally formed by pure gravity sedimentation in settling tanks and can be considered in the anaerobic process as a relatively easily degradable and as a readily dewaterable component in the entire sludge system.
The excess sludge substantially consists of bacterial mass which was substantially formed in the aerobic biological treatment stage and is relatively hard to degrade anaerobically compared with the primary sludge.
Owing to the strong colloidal system and the exopolymeric substances, the excess sludge possesses a high water binding capacity and is considered to be the component in the sludge digestion system which greatly decreases the degree of dewatering according to the proportion.
In the conventional wastewater treatment technique, the two types of sludge, primary sludge and excess sludge, are fed together to the anaerobic degradation stage (in the absence of air) and are digested to completion either mesophilically at 35 to 40° C., or thermophilically at 50 to 58° C. Mixing these sludges comprises two considerable disadvantages:    1. The primary sludge which is in itself readily dewaterable and readily degradable is adversely affected in its properties by an excess sludge proportion, which leads to poorer degradability and a reduction of the capacity in methane formation.    2. On the other hand, excess sludge, in the context of nutrient utilization, is a carrier of valuable material with respect to the possibility of utilizing carbon as methane and recovering as fertilizer the phosphorus which is contained in the excess sludge at 90%. The aim of recovering phosphorus from the excess sludge in a targeted manner can only be achieved economically and expediently if the phosphorus elimination is carried out in the previous wastewater treatment technique by what is termed the BioP method. In this method, no chemical precipitants are added for phosphate precipitation, but the property of the bacterial mass of accumulating polyphosphates to an increased extent under certain process conditions is utilized. Under anaerobic conditions, these polyphosphates are released again as orthophosphate (remobilization). This natural manner of ejecting phosphates from the wastewater opens up economic possibilities of phosphate recovery in the wastewater treatment technique. The phosphates, however, are contained at approximately 90% only in the excess sludge, whereas the primary sludge contains only the approximately 10 to 15% residual phosphates in another form.
During the anaerobic sludge conversion in what are termed the digestion vessels, the carbonaceous compounds are converted to methane (C4), carbon dioxide (CO2), ammonium (NH4) and water, wherein the methane is considered to be a valuable energy carrier and is usually utilized in combined heat and power stations for generating energy and heat. One aim in the optimization in the sludge treatment sector is the greatest possible conversion of the carbon compound in order to achieve maximum reduction in mass of the sludge and to generate maximum energy yield.
After completion of the anaerobic conversion of the mixture of primary and excess sludge, hitherto, after approximately 18 to 30 days, what is termed the digested sludge, using flocculation aids and dewatering machines (centrifuges, sieve belt presses, chamber filter presses etc.), is freed as far as possible from water since the dewatered wastewater sludge is generally used thermally or agriculturally. This is a considerable cost factor for municipalities. For the reduction in costs, attempts are therefore made to concentrate the sludges to the greatest possible extent.
For increasing the anaerobic degree of degradation of the excess sludge, pretreatment methods are known. These mechanical, thermal or else chemical methods by which the excess sludge is disrupted to a certain extent are termed disintegration methods, which can also be used in different combinations. These are used with the aim of disrupting the bacterial mass mechanically, thermally or else chemically to the extent that substrates are as accessible as possible to the anaerobic bacteria and the conversion rates and the conversion velocity are thus considerably increased. The disintegration methods are currently substantially used in the case of excess sludge, wherein after application of the disintegration method in the excess sludge, this is generally mixed again with the primary sludge and they are fed together to the anaerobic process.
The disintegration of wastewater sludge is described, for example, in N. Dichtl, J. Müller, E. Englmann, F. W. Günthert, M. Osswald: “Desintegration von Klärschlammein aktueller Überblick” [Disintegration of wastewater sludge—a current survey], in: Korrespondenz Abwasser 1997 (44), No. 10, pages 1726 to 1739. It is stated that, during the disintegration, the cell walls of the microorganisms in the wastewater sludge are destroyed and the cell contents liberated. The wastewater sludge can thereby be better aerobically or anaerobically stabilized, in such a manner that not only a decrease in the amount of sludge and the digestion time can be achieved, but also an increase in the digester gas production. The purely mechanical disruption of excess sludges leads only to a limited extent to an accelerated and more substantial degradation of the organic material in the digestion.
DE 199 40 994 A1 discloses, in addition, concentrating the predigested raw sludge using a decanter, in such a manner that the solid phase of the digested sludge is separated from a liquid phase.
DE 10 2004 023 171 A1 discloses a method for wastewater treatment in which the biological wastewater treatment is carried out without recirculating the treated excess sludge in the primary sludge. The excess sludge is withdrawn from the primary sludge in a sedimentation device downstream of the biological treatment.
By means of the separate treatment of the excess sludge with ozone and a subsequent aerobic oxidation in a suitable bioreactor, the reactors can be decreased in size and the energy consumption can be reduced.
In addition, EP 0 784 504 B1 discloses a method for hydrolyzing organic materials. By means of the hydrolytic pretreatment of wastewater sludge, breakdown of organic material to smaller particles is achieved, and so a high content of dry matter can be taken from the sludge. The nutrients, owing to the hydrolysis treatment, are in addition more readily accessible to bacteria and the sludge is pasteurized or sanitized.